In the utilization of a telephone system including two or more telephone sets connected to a common central office (C.O.) line, a user frequently will first take or initiate a call on one telephone set and then, for a variety of reasons, will subsequently move to another part of the house or office to continue the call on another telephone set. Alternatively, an incoming call may be for a third party, who may wish to take the call on a telephone set other then the one on which the answering party took the call initially. In such a telephone system, at least the telephone set on which a call is taken or initiated may include a pushbutton-activated HOLD circuit in order to maintain seizure of the common C.O. line after the handset is returned to its cradle, i.e., after the hookswitch is re-opened. Then, the call may be continued on a second telephone set connected to the same C.O. line; or the call may be re-established on the first telephone set; or the caller might hang up the handset of the telephone set. Under each of these three conditions, the C.O. line should be released from the HOLD condition.
An appropriate impedance termination is connected across the tip and ring telephone wires when placing a call on hold. This impedance termination is required to maintain current flow to the central office equipment when the telephone handset is returned to its cradle. The voltage developed across the hold impedance termination will vary as a function of the central office voltage, the plant and loop resistance and the resistance of the hold impedance termination itself. The voltage and current at the telephone may also be momentarily interrupted by the central office equipment in order to accomplish certain signalling functions and reconfigurations. The loop interruptions will vary in time, may have a specific signalling purpose for the telephone, or may simply be an intermittent result of plant reconfiguration. Furthermore, the voltage developed across the hold termination will change as an associated extension telephone goes off-hook. The amount of change also is a function of loop parameters such as voltage and resistance. The HOLD and HOLD release circuit must, therefore, adjust to and operate over a wide range of loop conditions while distinguishing between unintentional and intentional signals or loop changes. The circuit preferably derives its operating power directly from the telephone line without requiring external sources of power.